Well at least Slack doesn't even offer such stupid policies It does (indirectly) if you use SAML single sign on (which many large corporations do). That is wha

Well at least Slack doesn't even offer such stupid policies It does (indirectly) if you use SAML single sign on (which many large corporations do). That is what is happening with Teams. You're not being asked to sign in to Teams itself. You're being asked to identify yourself via Microsoft single sign on and the people that set up that companies Microsoft single sign on have set SMS as the default 2FA. Whether or not you should use SMS for 2FA is of course a separate discussion.